Actuators are known that have additional safety levels constituted by the presence of a second nut which, being separated from the screw by a small amount of clearance, takes up the load on the primary nut in the event said primary nut fails.
While admittedly offering safety, such devices suffer from the risk that a mode of operation relying on the second nut only, after the first nut has failed, might not be detected. A device in such a situation no longer has its additional safety level, and therefore loses its initial advantage.
It is desirable for that mode of operation to be indicated as quickly as possible in order to avoid a dormant failure mode.
By way of illustration, it has also been proposed, in U.S. Pat. No. 5,833,033, in the field of detecting wear in brake pads in motor vehicles, to provide a wear detector probe in the form of an electrical cable loop organized to be cut by abrasion. The electrical link being broken indicates that it is necessary to replace the brake pad. That type of device, proposed in another field, does not satisfy the constraint of exactly detecting the precise instant at which the stresses are transmitted to the secondary nut.